It is widely practiced to seal electronic devices, for example, transistors, diodes, ICs, capacitors and the like with a synthetic resin to retain electrical insulating property and avoid variations in physical properties due to the surrounding atmosphere while enjoying advantages in productivity and cost.
Thermosetting resins such as epoxy resins and silicone resins have conventionally employed as synthetic sealing resins. They are however said to have drawbacks such that they require a long time for thermosetting and hence a long molding cycle, their storage is not easy because of the need for prevention of the progress of curing, and sprues and runners cannot be reused.
Therefore, it has recently been proposed to use, as electronic device sealing resin compositions, resin compositions composed principally of a polyphenylene sulfide (hereinafter abbreviated as "PPS") which is a thermoplastic resin having excellent heat resistance and flame retardance. Reference may be had, for example, to Japanese Patent Publication No. 9014/1981, Japanese Patent Application Laid-Open No. 17153/1982, Japanese Patent Application Laid-Open No. 21844/1982, Japanese Patent Application Laid-Open No. 40557/1982, Japanese Patent Application Laid-Open No. 20910/1984, Japanese Patent Application Laid-Open No. 20911/1984, Japanese Patent Publication No. 40188/1985, Japanese Patent Application Laid-Open No. 65351/1987, Japanese Patent Application Laid-Open No. 197451/1987, and Japanese Patent Application Laid-Open No. 146963/1988).
A PPS resin however has inferior adhesion to lead frames or bonding wires of an electronic device. When sealing is conducted with a PPS resin composition and the resultant sealed electronic device is placed in a high-humidity atmosphere, moisture tends to penetrate through an interface between the sealing resin and the lead frames or bonding wires so that the electrical insulating property may be reduced and/or the lead frames and wires may be corroded and the electrical characteristics of the electronic device may be reduced accordingly.
An electronic device is mounted on a circuit board. Its bonding to the board is achieved by a solder. Reflecting the recent diversification of board-mounted devices in both configurations and dimensions, soldering apparatus increasingly employ a temperature higher than conventional temperatures to ensure successful bonding of larger devices to a board. As a result, smaller devices which are mounted on the board concurrently with the larger devices are increasingly required to have far higher solder heat resistance compared to the larger devices since the smaller devices have a relatively smaller heat capacity. For example, with respect to devices for which the heat resistance of about 260.degree. C. as measured by a method to be set out subsequently has heretofore been considered sufficient, there is a recent and increasing trend toward the need for durability against higher temperatures. Otherwise, sealing materials inside such electronic devices may be deteriorated or interfacial separation and cracks may occur between the resin portions and the lead frames, so that the electronic devices may no longer be suitable for use.
A sealed electronic device is required to have excellent moisture resistance and solder heat resistance as described above. Where conventional PPS resin compositions are used as sealing resin compositions, their moisture resistance and solder heat resistance are still insufficient. This has remained as a problem for their actual use.